Three half-sandwiched iron(II) monocarbonyl complexes with PNP ligands: Their chemistry upon reduction and catalysis on proton reduction

Three half-sandwiched iron(II) monocarbonyl cationic complexes ( 1 + − 3 + ) containing a azadiphoshine ligand (PNP, L 1 – 3 ) and an iodide as the counter anion, [Fe(II)Cp(CO)( L 1 – 3 )]Ι (Cp =  η 5 -cyclopentadienyl; L 1  = Ph 2 PN(cyclohexyl)PPh 2 , 1 + ; L 2  = Ph 2 PN(phenyl)PPh 2 , 2 + ; L 3  = Ph 2 PN(benzyl)PPh 2 , 3 + ) were reported. All the complexes were fully characterized, of which complexes 2 + and 3 + were crystallographically analysed to confirm their “piano stool” geometry. Electrochemically, complexes 1 + − 3 +  exhibited two major reduction events at about −1.9 V and −2.3 V, respectively, assigned to Fe(II) → Fe(I) and Fe(I) → Fe(0), respectively. The first reduction produces a 19e-species, [Fe(I)Cp(CO)( L 1 – 3 )] ( 1, 2 , and 3 ) whereas the second event is not assigned to the further reduction of the 19e-species but a 17e-intermediate ([Fe(I)Cp(CO)( m - L 1 – 3 )]; 1′, 2′ and 3′ ) which is chemically generated from the 19e-species by cleaving one of the two Fe−P bonds. However, the 19e-species 3 is considerably more stable than the other two analogues ( 1 and 2 ) due to H − π interaction between the ortho -H atoms from one of the four phenyl rings on the phosphine atom and the phenyl ring of the benzyl group as revealed by theoretic calculations. Therefore, species 3 is stable enough for the reduction of 3  →  3 − to be electrochemically detected right before the second major reduction. To further shed some light on the electrochemical mechanism, both bulk electrolysis and chemical reduction of complexes 1 + − 3 +  with cobaltocene as the reducing agent were conducted. Under N 2 atmosphere, the neutral 17e-species, [Fe(I)Cp(CO)( m - L 1 – 3 )] ( 1′ - 3′ ) was spectroscopically detected whereas under CO atmosphere, CO-adducts, [Fe(I)Cp(CO) 2 ( m - L 1 – 3 )], were generated. The formation of CO-adduct explains the loss of reversibility of the first reduction ( 3 + ) under CO atmosphere in its electrochemistry. With acetic acid as the proton source, only the second major reduction of the complexes exhibited significant catalytic activity, of which complex 1 + possessed the best performance. Significant proton-coupled electron transfer (PCET) effect was observed for complex 1 + due to the strong electron-donating capability of the ligand ( L 1 ).